This invention relates to a method of reproducing a magneto-optical medium, and more particularly to a method of reproducing a magneto-optical disc, which can record/reproduce information at high density.
A magneto-optical disc which can rewrite an information signal includes a magnetic thin film. By irradiating a laser beam to this film to heat it, the direction of magnetization at the heated portion (recording bit) is caused to be in correspondence with an external magnetic field corresponding to recording information, thus to carry out recording of information. On the other hand, in the case of reproduction, a laser beam is irradiated onto a track of the recording bit to carry out reproduction by making use of the Kerr effect that the plane of polarization of a reflected light rotates by the direction of magnetization. In the case where a magneto-optical disc is of a structure including two layers or more having a reflection film in addition to the magnetic thin film, the Faraday effect is also utilized.
The track recording density of information on a magneto-optical disc is determined by the carrier to noise ratio, i.e. C/N of a regenerative signal. In a conventional typical magneto-optical recording and reproduction, as shown in FIG. 1A, the entirety of the area of a beam spot 1 which is a light irradiation area by a laser beam on a magneto-optical disc is caused to be a regenerative signal detection area. For this reason, the reproducible track recording density is determined by the diameter of the laser beam spot.
For example, if the diameter d of the laser beam spot 1 is smaller than the pitch .tau. of recording bits 2 as shown in FIG. 1A, there is no possibility that two recording bits fall within the spot 1. In this case, the reproduced output waveform is as shown in FIG. 1B. Thus, a regenerative signal can be read. In contrast, in the case where recording bits are formed at high density and the diameter d of the laser beam spot 1 is greater than the pitch .tau. of the recording bits 2 as shown in FIG. 1C, two recording bits 2 concurrently fall within the spot 1, so the reproduced output waveform is fixed as shown in FIG. 1D. For this reason, it is impossible to separately reproduce those two recording bits, resulting in reproduction unable state.
The spot diameter d is dependent upon the wavelength .lambda. of a laser beam and the numerical aperture NA of an object lens. In view of this, in the prior art, a device or scheme is made such that a laser beam having a short wavelength .lambda. is used, or the numerical aperture NA of the object lens is increased to thereby reduce the spot diameter d of the laser beam, thus to allowing the recording density to be high. However, such a scheme has a limit because of the problem of a laser light source and/or the problem of an optical system. Therefore, this is an obstacle to implementation for allowing the recording density to be higher.
Further, the track density is also limited by crosstalk mainly from adjacent tracks. In the case of the prior art, a quantity of this crosstalk is also dependent upon the laser beam spot diameter d. Similarly, this is an obstruction to realization of high density recording.
The applicant of this application has already proposed a magneto-optical disc and a method of reproducing the same such that the readable track recording density and the track density are permitted to be high without alteration of the laser beam spot diameter.
In accordance with one system thereof, as described in U.S. Pat. No. 5,018,119, a magneto-optical disc including a multi-layer film comprised of a recording layer 3, an intermediate layer 4, and a reproduction layer 5 formed in a stacked manner as shown in FIG. 2A. The recording layer 3, the intermediate layer 4 and the reproduction layer 5 are constituted by materials having the Curie temperatures of 300.degree. C., 120.degree. C. and more than 400.degree. C., respectively.
In the case of this magneto-optical disc, in the state of a room temperature prior to reproduction, the recording layer 3, the intermediate layer 4 and the reproduction layer 5 are magnetically coupled in a state of magnetostatic coupling or exchange coupling as shown in FIG. 2A, and the magnetic polarities of the recording bits of the recording layer 3 are all transferred to the reproduction layer 5. In the figure, arrows indicate directions of magnetization.
At the time of reproduction, as shown in FIG. 2B, a laser beam 6 is irradiated on the magneto-optical disc, and a predetermined reproducing magnetic field Hre is applied thereto. In the magneto-optical disc, as shown in FIG. 2C, by irradiation of the laser beam 6, there takes place a region 8 in which the temperature in the layers 3, 4 and 5 becomes equal to more than the Curie Point of the intermediate layer 4. At this time, since the magneto-optical disc rotates at a high speed, this high temperature region 8 would be a region shifted in a rotation direction by a quantity corresponding to a moving velocity (linear velocity) of the magneto-optical disc with respect to the position of a scanning spot 7 of the irradiation laser beam 6.
In the high temperature region (mask region) 8, since the temperature of the intermediate layer 4 is above its Curie point Tc, the magnetic property of the intermediate layer 4 is lost as shown in FIG. 2B. Thus, the magnetic coupling between the recording layer 3 and the reproduction layer 5 at the portion of this region 8 is annihilated. As a result, magnetization of the reproduction layer 5 is in correspondence with the direction of the reproducing magnetic field Hre. Namely, recording bits of the reproduction layer 5 in the high temperature region 8 are erased. Thus, a region 9 except for the region overlapping with the high temperature region 8 of the region of the scanning spot 7 serves as a reproduction region. Namely, the scanning spot 7 of a laser beam is partially masked by the high temperature region 8. Thus, a small area which is not masked serves as the reproduction area 9. Note that this small area is smaller in diameter than the pitch between the magnetic bits 2.
Since reproduction of bits is carried out by detecting a Kerr rotational angle of a reflected light from the small reproduction area 9 where the scanning spot 7 of a laser beam is not masked by the mask region 8, there results the effect equivalent to the fact that the spot diameter d of the laser beam spot 7 is reduced. Thus, the track recording density and the track density can be improved.
The above-mentioned reproducing method is called a reproducing method of the erasing type.
Further, the applicant of this application has also proposed another system. This system is described in the Japanese Patent Application No. 229395/89 (corresponding to U.S. Pat. No. 5,168,482).
A magnetic thin film of a magneto-optical disc of this system is in principle comprised of a stacked film of a recording film and a reproduction layer. In this case, the recording layer and the reproduction layer can be subjected to magnetostatic coupling or magnetic exchange coupling. The Curie point of the reproduction layer is lower than that of the recording layer.
In accordance with this system, an approach is employed in principle to apply an initialization magnetic field to a magneto-optical disc prior to reproduction to allow the direction of magnetization of the reproduction layer to be in correspondence with the direction of the initialization magnetic field, thus to erase recording bits of the reproduction layer. The magnitude of the initialization magnetic field Hin is set to a value greater than the magnitude of a magnetic field Hcp required for reversing the polarity of the magnetization of the reproduction layer (Hin&gt;Hcp), and sufficiently smaller than the magnitude of a magnetic field Hcr required for reversing the polarity of the magnetization of the recording layer (Hin&lt;&lt;Hcr).
In the case of reproduction, in a state initialized as described above, a laser beam is irradiated onto the magneto-optical disc. In the same manner as in the previously described case, the disc temperature of a region (corresponding to the region 8 of FIG. 2C) shifted with respect to the scanning spot position in a rotation direction in dependency upon a rotation moving velocity (linear velocity) of the magneto-optical disc becomes higher than a predetermined temperature Ts. Thus, since the coercive force in that region of the reproduction layer becomes small, the magnetization of the recording bits are transferred only into the region of the reproduction layer in which the temperature is higher than the predetermined temperature Ts. By detecting a Kerr rotation angle of a plane of polarization of a reflected light from a region overlapping with a laser beam spot of the recording bits area, reproduction is carried out.
In the case of this system, the region except for the region, which has a temperature higher than the predetermined temperature Ts of the region of the scanning spot of a laser beam, is a so called a mask region where no recording bit appears. The portion where the high temperature region and the beam spot region overlap with each other serves as a reproduction region. Since this region is both smaller than the spot diameter and the pitch between the bits, the track recording density and the track density can be caused to be high.
It is to be noted that, in practice, in order to stably hold the initialized state of the reproduction layer and to satisfactorily carry out transfer of the magnetization directions of the recording bits from the recording layer at the time of reproduction, a magneto-optical film of four layers as shown in FIG. 3 is formed in the disc.
Namely, the magneto-optical disc includes a stacked film of four layers of a recording layer 11, an intermediate layer 12, a reproduction auxiliary layer 13, and a reproduction layer 14. The Curie temperatures of the recording layer 11, the intermediate layer 12, the reproduction auxiliary layer 13, and the reproduction layer 14 are set to, e.g., 250.degree. C., 250.degree. C., 120.degree. C. and more than 300.degree. C., respectively.
The recording layer 11 is the layer for holding recording bits without being affected by the initialization magnetic field, the reproduction magnetic field, or the reproduction temperature, etc., and has a sufficient coercive force at a room temperature or a reproduction temperature Ts.
The vertical anisotropy of the intermediate layer 12 is small as compared to those of the reproduction auxiliary layer 13 and the recording layer 12. For this reason, a magnetic domain wall formed between the reproduction layer 14 and the recording layer 11 stably exists in the intermediate layer 12. For this reason, the reproduction layer 14 and the reproduction auxiliary layer 13 stably maintain an erased state (initialized state).
The reproduction auxiliary layer 13 serves to enhance the coercive force of the reproduction layer 14 at a room temperature. For this reason, magnetization of the reproduction layer 14 and the reproduction auxiliary layer 13, of which directions are caused to be in correspondence with each other by the initialization magnetic field, stably exists even if magnetic domain walls exist. Further, the coercive force of the reproduction auxiliary layer 13 abruptly becomes small at about the reproduction temperature Ts at the time of reproduction. For this reason, magnetic domain walls confined within the intermediate layer 12 extend to the reproduction auxiliary layer 13 to finally reverse the reproduction layer 14, thus allowing the magnetic domain walls to disappear. By this process, the magnetization directions of the recorded bits is transferred to the reproduction layer 14.
The magnetization reversal magnetic field Hcp of the reproduction layer 14 is small even at a room temperature, and magnetization is easily reversed. For this reason, the directions of magnetization at the entire surface of the reproduction layer 14 are in correspondence with each other. The magnetization of which directions are caused to be the same is supported by the reproduction auxiliary layer 13, and stable state thereof is kept even in the case where magnetic domain walls exist between the reproduction auxiliary layer 13 and the recording layer 11. As previously described, at the time of reproduction, magnetic domain walls between the reproduction auxiliary layer 13 and the recording layer 11 disappear. Thus, the magnetization directions of the recording bits in the recording layer 11 are transferred to the reproduction layer 14.
In carrying out an actual reproduction, initialization of the reproduction layer 14 and the reproduction auxiliary layer 13 is carried out by the initialization magnetic field Hin prior to reproduction as shown in FIG. 4A. At this time, magnetic domain walls (indicated by arrows in a lateral direction in FIG. 4A) stably exist in the intermediate layer 12, thus, the reproduction layer 14 and the reproduction auxiliary layer 13 stably maintain the initialized state.
Then, as shown i n FIGS. 4B and 4C, a laser beam 15 is irradiated onto a track of a recording bit, and a reproduction magnetic field Hre is applied thereto. For this reproduction magnetic field Hre, a magnetic field having strength greater than that of a magnetic field for reversing the reproduction layer 14 and the reproduction auxiliary layer 13 and for allowing magnetic domain walls of the reproduction auxiliary layer 13 to disappear is required. However, this magnetic field is required to have a strength limited to such an extent that the reproduction layer 14 and the reproduction auxiliary layer 13 do not reverse their directions of magnetization.
By temperature elevation by irradiation of the laser beam 15, in the same manner as previously described, at the portion shifted in a rotation direction of the magneto-optical disc with respect to the beam scanning spot 16, there is created a high temperature region 17 where the temperature is above the reproduction temperature Ts. As a result, the coercive force at the portion of the reproduction auxiliary layer 13 in that region 17 (the portion to which slanting lines are attached in FIG. 4C) is lowered. Since the reproduction magnetic field Hre is smaller than the exchange coupling forces between the recording layer 11 to the reproduction layer 14, magnetic domain walls at that portion disappear. As a result, the magnetization directions of the recording bits of the recording layer 11 are transferred to the reproduction layer, so corresponding bits are produced in the reproduction layer 14. Thus, the region 18 overlapping with the high temperature region 17 of the region of the scanning spot 16 serves as a reproduction region in substance. Namely, the region except for the region 18 overlapping with the high temperature region 17 of the region of the scanning spot 16 of a laser beam is masked. Thus, this overlapping region 18 serves as a reproduction region.
Since reproduction of bits is carried out by detecting a Kerr rotation angle of a reflected light from the small reproduction region 18 where the scanning spot 16 of a laser beam and the high temperature region 17 overlap with each other, there results the effect equivalent to the fact that the spot diameter d of the laser beam spot 16 is reduced to less than the pitch between the bits. Thus, the track recording density and the track density can be caused to be high.
The above-mentioned reproducing method is called herein a reproducing method of the high temperature region type.
In a manner stated above, the track recording density and the track density can be caused to be high without reducing the diameter of the laser beam scanning spot. However, even if the external reproduction magnetic field is fixed and the laser beam power is fixed, the dimensions of the reproduction regions 9 and 18 in the respective reproducing methods would vary in dependency upon a temperature change or a linear velocity change of the magneto-optical disc.
For example, in the reproducing method of the erasing type, in the case where the temperature of a magneto-optical disc is high, the temperature distribution state is such that it shifts in a higher temperature direction as indicated by the curve 21 of FIG. 5B. For this reason, the high temperature mask region above the Curie temperature Tc becomes a region 22 as shown in FIG. 5A. Thus, the substantial reproduction region 9 is reduced.
Further, in the case where the temperature of the magneto-optical disc is low, the temperature distribution state is such that it shifts in a lower temperature direction as indicated by the curve 23 in FIG. 5B. For this reason, the high temperature region above the Curie temperature Tc becomes a region 24 of FIG. 5A. Thus, the substantial reproduction region 9 becomes large.
On the other hand, in the case of the high temperature type, as apparent from the principle, in the case where the temperature of the magneto-optical disc is high, the reproduction region becomes large, while in the case where the magneto-optical disc temperature is low, the reproduction region becomes small.
Further, for example, in the reproducing method of the erasing type, in the case where the linear velocity is low, the transit time of the scanning spot 7 per unit movement distance is prolonged. For this reason, as indicated by the curve 25 in FIG. 6B, the temperature distribution state by laser beam irradiation is such that the high temperature mask region above the Curie temperature Tc becomes broad as indicated by the region 26 in FIG. 6A. Thus, the substantial reproduction region 9 becomes small.
In contrast, in the case where the linear velocity is high, since the transit time of the scanning spot 7 per unit movement distance is short by indicated by the curve 27 in FIG. 6B, the temperature distribution state by laser beam irradiation is such that the high temperature region above the Curie temperature Tc is narrowed as indicated by the region 28 in FIG. 6A, the substantial reproduction region 9 becomes large.
On the other hand, in the case of the high temperature type, as apparent from the principle thereof, in the case where the linear velocity of the magneto-optical disc is low, the reproduction region becomes large, while in the case where the linear velocity of the magneto-optical disc is high, the reproduction region becomes small.
As understood from the above discussion, if the temperature of the magneto-optical disc changes by a changes of an environmental temperature therearound, or the linear velocity of the magneto-optical disc varies in dependency upon the reproducing position, the dimension (area) of the substantial reproduction region 9 or 18 at the time of reproduction would vary. For this reason, a stable reproduction having good C/N cannot be conducted.
In addition, in the case where the track recording density of the magneto-optical disc changes, it is advantageous for carrying out reproduction of satisfactory C/N that the dimension of an optimum reproduction region is varied in dependency upon the track recording density.
Namely, in the reproducing method of the erasing type or the high temperature type, in the case where the track recording density is low, the pitch interval of recording bits is long. For this reason, in order to carry out reproduction having good C/N, it is desirable that the reproduction region 9 or 18 is large. On the other hand, in the case where the track recording density is high, the recording bit interval become short. For this reason, in order to carry out reproduction of good C/N, it is desirable that the reproduction region 9 or 18 is narrow. Accordingly, even if information is recorded on a magneto-optical disc at a fixed track recording density, in the case where the track recording density varies according to the kind of magneto-optical discs, it is advantageous to effect a control to allow the dimension of the reproduction region 9 or 18 to take an optimum value in dependency upon the track recording density.